PALEOCLIMATIC RECONSTRUCTION OF LATE EOCENE MYANMAR VIA STABLE ISOTOPE PROXIES

The Asian monsoons affect the lives of one third of the human population, yet the controls on monsoon intensity and longevity are not fully understood. Recent studies have highlighted that the South Asian monsoon is as old as the Eocene; early monsoonal intensity has been proposed to be driven by variations in atmospheric pCO₂.

This study proposes to trace the response of the South Asian monsoon to decreasing pCO₂ during the Eocene Oligocene Transition (EOT) in the sedimentary archives of central Myanmar. To do so, we present new stable isotope data from the upper Eocene – lower Oligocene Yaw Formation in a preliminary approach to reconstruct late Eocene monsoonal intensity.

The two sources of isotope data are fossil freshwater shells and lignite layers. Pristine, unaltered bivalve and gastropod shells – confirmed with cathodoluminescence and scanning electron microscopy – were subsampled and analyzed for δ¹⁸O values. Lignite samples were analyzed for bulk organic matter δ¹³C content. The oxygen isotope data provides insight into annual rainfall seasonality and overall monsoon rainfall intensity, while the carbon isotopes record vegetation types and temporal shifts in ecosystems. These preliminary data are the first step in reconstructing the evolution of South Asian monsoonal intensity during the EOT, and will be followed by further investigation of younger sedimentary units in central Myanmar.